In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles. Many of the electrical components share a common direct current (DC) voltage bus. The DC voltage bus (alternatively referred to as a high voltage DC bus, a power bus, or an electrical bus) is configured to carry DC power from one or more DC power sources in the vehicle to numerous other electrical components in the vehicle, such as power inverters, air compressors, power converters, auxiliary power modules, and the like.
A fuel cell (or fuel cell stack) is one DC power source that may be connected to the DC voltage bus for providing DC power to other electrical components in the vehicle. Due to internal resistances and/or ohmic losses and other operating characteristics of the fuel cell, heavy loading conditions and/or startup of the vehicle often requires power in excess of what the fuel cell can provide. Accordingly, a battery (or rechargeable battery pack) is often employed to provide supplemental power during heavy loading conditions and/or startup as well as to store regenerative energy from the drive unit of the vehicle. However, many batteries are unable to accommodate large voltage swings of the fuel cell during operation. Some prior art systems utilize a bi-directional DC/DC converter to step up the voltage from the battery to match the DC voltage bus voltage dictated by the voltage output of the fuel cell, or alternatively, to step down the fuel cell voltage during battery recharging. DC/DC converters are relatively large, costly and heavy, providing obvious disadvantages.